Various types of optical analytical instruments have been developed which direct analytical radiation, e.g., infrared, to an optical system which focuses the beam of radiation on a sample. The radiation transmitted through or reflected from the sample is then directed by further optics to a detector. Such optical systems are particularly used in infrared spectrometers.
In an optical system employed in infrared spectroscopy, the radiation beam from the source is typically collimated and directed to a focusing element which focuses the beam on the sample. If the radiation transmitted through the sample is to be analyzed, another focusing element is mounted on the opposite side of the sample to collect the radiation that is passed through the sample and to collimate it. The collimated beam may then be deflected one or more times before being directed to a focusing element which focuses the beam on the detector. If, however, the reflected radiation from the sample is to be analyzed, a separate optical system is used in which the radiation reflected from the sample is collected and directed to and focused upon a detector. Such dedicated reflectance systems are relatively expensive and thus have had limited use. Additionally, to test a single sample for both transmission and reflection, the sample must be moved from one optical system to the other and the optics properly aligned, a time consuming task. Because two separate optical systems are used, it has been difficult to accurately analyze the transmission and reflection characteristics of the same sample under comparable conditions to determine the absorption of the sample.